1. Field of the Invention
This invention relates to internetworking between wireless local area networks (WLANs) and wide area mobile wireless networks.
2. Discussion of Related Art
Subscribers are adopting wireless telephony in increasingly large numbers. This trend is being fueled further by attractive rate plans that are bringing the cost of telephony to acceptable levels for people in most walks of society. It is more common for people to rely almost exclusively on a mobile telephone for their telephony needs. In an office or enterprise environment, however, mobile telephony has not surpassed wireline enterprise telephony for a number of reasons, salient amongst which are the following:                The weakness of R/F signals from the wide area network infrastructure within an office building, leading to problems in mobility management and voice quality.        The availability of special PBX features in office telephony systems, such as abbreviated dialing, offer a strong incentive for the continued use of enterprise telephony solutions.        
The situation at the present is that people employ two distinct telephony systems, one in the wireless wide area network, and another in the enterprise premises that is a wireline telephony system, leading to a plurality of handsets, voice mailboxes and addressing mechanisms.
Parallel to these developments, wireless local area networks are becoming increasingly popular for wireless data applications. In such networks, reasonable bandwidth is available to enterprise-wide wireless clients, e.g., in 802.11b WLAN networks up to 11 Mbps are available to a wireless client. This bandwidth is more than sufficient to carry voice as well. Moreover, widespread use of WLAN technology is driving down the price of the technology.
FIG. 1 shows an exemplary wireless wide area network (WWAN) 100 (also known as a wide area mobile wireless network). WWAN 100 includes a plurality of remote units (handsets) 102 in radio contact with one or more antennae based systems called Base Transceiver System (BTS) 104 that transceive the radio signals to/from the handsets. A plurality of BTS communicate with a controller called the Base Station Controller (BSC) 106 via fixed links 108 using a variety of protocols and techniques, such as TDM, IP etc. A plurality of BSC communicate with a switch called the mobile switching center (MSC) 110 that provides connectivity to a wide area switched telephone network (WSTN) 111. The WSTN includes signaling links 113, such as SS7 links, and the public switched telephone network (PSTN) 112. As illustrated by MSC 115, an MSC may include a control plane 117 for handling messages on the signaling links, which may be communicated according to a variety of protocols, such as IOS, GSM A interface, IS 41, GSM MAP, etc. The MSC 115 may also include a media gateway 119 that cooperates with the control plane for handling the bearer circuits of the PSTN 112. Some modem MSCs, such as MSC 115, may also communicate on IP networks, such as IP network 120.
An MSC with its associated BSC and BTS collectively define a coverage area in which handsets are allowed to receive or transmit telephone calls. Incoming calls to a mobile handset arrive from the PSTN to a gateway MSC, e.g., 115, that then routes the call to the MSC 110, called a serving MSC, within whose coverage area the receiving handset is currently roaming. Outgoing calls from a handset are routed to the serving MSC 109 of the originating handset from where the call is either routed to the serving MSC of the receiving mobile handset via the gateway MSC or to the WSTN via the gateway MSC from where the WSTN routes the call to a (wireline) handset.
In some arrangements, the gateway and serving MSC functions may be implemented by the same physical entities. Subscribers are allowed to roam in the coverage area and while roaming the various entities of the WWAN cooperate to ensure that the wireless connectivity of the handset is preserved under roaming. A handset may roam from the coverage area of one set of BTS/BSC/MSC to the coverage area of another set of BTS/BSC/MSC. The former set of BTS/BSC/MSC is called the source and the latter set is called the target entities. A set of procedures has been defined that mediate the handoff of the handset from the source to the target entities of the WWAN. As a consequence of the handoff procedures, an update of the location of the handset may occur. This is accomplished by the handset sending a location update message to an MSC that routes the message to a registry called the Home Location Register (HLR) 114 using standard industry protocols such as IS-41, GSM-MAP, etc.
Various air interface technologies are used for the communication between handsets 102 and BTS 104. These technologies include code division multiple access (CDMA), global system for mobile communications (GSM), Personal Digital cellular (PDC), etc., and various extensions and enhancements of these technologies such as CDMA2000, universal mobile terrestrial system (UMTS), international mobile telephone IMT-2000, etc. All such networks employ the above referenced entities in well known, albeit using different nomenclature, configurations to transceive telephone calls. All this is well known to practitioners with ordinary skill in the art.
FIG. 2 illustrates an exemplary wireless local area network (WLAN) 200. WLAN includes one or more geographical areas (cells) called basic service set (BSS) 202. A cell is controlled by a system called an access point (AP) 204. Typically, a WLAN includes several BSS, each with its associated AP. The AP are interconnected usually with a wireline network 206 typiqcally using Ethernet in 802.x WLAN technologies. The AP communicate with an enterprise router 208 that typically routes traffic within and out of an enterprise network. Wireless data clients 210 are allowed to roam within a defined BSS and across the defined BSS, with handoff of the client from one AP to the adjoining AP in accordance to known procedures. In typical WLAN implementations, the physical layer uses variety of technologies, e.g., in 802.11 WLAN implementations the physical layer may use infrared, frequency hopping spread spectrum in the 2.4 GHz Band, or direct sequence spread spectrum in the 2.4 GHz Band. The medium access layer (MAC) in addition to carrying out typical functions performs additional functions such as packet fragmentation, re-transmission and acknowledgements.
The MAC layer supports two basic access mechanisms: the distributed coordination function (DCF) and the point coordination function (PCF). In DCF the basic access mechanism is a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. A typical example is Ethernet that is a CSMA with collision detection (CD) mechanism. In CSMA protocols a client wishing to transmit senses the medium, and if the medium is found to be busy, i.e., is being used by some other client, defers the transmission; otherwise it is allowed to transmit. There is always a possibility that two clients will sense the medium to be free and start transmissions thus resulting in collisions; therefore, collision avoidance and detection is very important in such protocols. For example, the 802.11 WLAN uses collision avoidance and detection mechanisms. An 802.11 client wishing to transmit senses the medium and if found busy defers the transmission; otherwise, it transmits. The receiver checks the receipt of a proper transmission (via the cyclical redundancy check—CRC) and if found satisfactory, sends back an acknowledgement. Receipt of the acknowledgement will indicate to the transmitter that the transmission was received properly. If no acknowledgement is received, the transmitter will retransmit until an acknowledgement is received or the transmitter decides to abort the transmission. In order to further reduce the possibility of collisions certain implementations also use the virtual carrier sense mechanism. In this scheme, a client wishing to transmit, first signals its intent by sending a request to send (RTS) to the intended receiver. The receiver responds with a clear to send (CTS) that effectively “reserves” the medium for the transmitter and receiver. The transmitter may now transmit the intended information.
In those cases when a client wishing to transmit finds the medium busy, the client defers the transmission. The client is thus obliged to re-try to find the status of the medium. In standard approaches to this problem, an “exponential back off procedure” is used to determine the frequency of re-trials. The method involves the choice of a random number and awaiting that many time slots before a re-trial. If a re-try finds the medium busy again, the re-trial number is reduced exponentially. This back off procedure is also used after a successful transmission and after each re-transmission.
When a client wishes to access a BSS (either after a power up or when first entering the BSS) it needs to get synchronization information from the AP controlling the BSS. Two methods have been defined for clients to get this information. In the passive scanning method the client waits to receive a “beacon frame” from the AP that is transmitted by the AP at regular intervals. The beacon frame contains the synchronization information. In the second method, called active scanning, the client sends a probe to the AP and awaits a response to the probe. Once the station finds an AP, it needs to be authenticated. This requires exchange of information between the AP and the client to establish the authenticity of the client. Once the authentication process is completed, the client starts the association process that involves exchange of information between the client and the APs about the location of the client and the capabilities of the BSS. At the completion of the association process, the client is ready for receiving or transmitting data.
In the PCF access mechanism, the AP gains control of the medium upon sensing it to be free for a given length of time called the point inter frame space (PIFS). The AP then assumes the role of the coordinator and starts to poll all stations enumerated on a “poll list” maintained by the AP. When polled, a station is allowed to transmit. The period in which the AP supports PCF mode is contention-free so may provide better opportunities for voice traffic. The AP must alternate the DCF and PCF periods. All this is well known to practitioners with ordinary skill in the art.
Therefore, what is needed is a method for internetworking of WLAN and Wireless Wide Area Networks (WWAN) for voice communications with full mobility management across the two networks and the preservation of PBX features in the WWAN environment.